Is Less Inertia More?

A lot has been said in recent times, especially following the “system black” event in SA on 28 Sept 2016, about the perils of moving too rapidly to grid systems with reduced inertia, ie more renewable energy generators and less fossil fuel fired generator sets.

We might seek to understand this issue by thinking of the difference between controlling the speed of a truck (high inertia) verses a motor bike (low inertia). Assuming a constant accelerator setting, ie constant amount of fuel being consumed, the truck speed will be less affected by speed bumps or hitting something on the road; than a motor bike. When this analogy is translated to the electricity grid, the argument is that a grid with a high percentage of generators with high inertia, is better able to “ride through” faults and maintain a constant speed. “Speed” translates to frequency on the grid as we are talking about rotating machines where their rotational speed determines the frequency of the alternating current electricity being supplied.

This overly simplistic line of argument overlooks the fact that the primary determinant of the speed of the truck or motor bike is in fact the accelerator setting, ie the amount of fuel being fed into the engine. Another way of saying this is that the speed of the vehicle is determined by the governor which in turn controls the amount of fuel being supplied to the engine.

The interesting aspect of the analogy that is not often recognised is that it is easier to adjust the speed of the motor bike than the truck should there be a need to do so. The lower inertia of the motor bike means it is much more responsive, in terms of adjusting its speed, to changes in the accelerator setting than a truck.

Large turbine driven generator sets also have the equivalent of a governor that monitors the load on the generator and adjusts of the gas or steam flow to maintain a constant rotating speed and hence constant frequency. If the speed or frequency falls or rises beyond an acceptable range, it is easier to adjust the rotational speed of a low inertia generator or series of generators in a grid, than a grid with high inertia.

Converse to some popular opinions that low inertia grid systems (significant percentage of renewable generators) are difficult to control when faults occur, with appropriate “governor” or control systems, they can be adjusted more quickly than a system with high inertia. In terms of the analogy, a change in the accelerator setting adjusts the speed of a motor bike more quickly than a truck.

A related issue is the fact that wind turbines up to now have nearly all been configured to operate as if they have no inertia, ie they follow frequency variations and offer no resistance to frequency changes in the grid caused by external factors. In terms of the analogy, akin to a bicycle or even just a feather blowing in the wind.

Both wind turbines and batteries can be configured to provide frequency support or resistance to unwanted frequency changes. The magnitude of the support available will be determined by the size and number of wind turbines and batteries.